1. Field of the Invention
The present invention relates to a liquid crystal display device, and more particularly, to a liquid crystal display device which can reduce driving power for cell selection and a method for driving the liquid crystal display device.
2. Description of the Related Art
In a liquid crystal display device which uses an STN (Super Twisted Nematic) type liquid crystal panel, pixel driving signals, that is, driving signals for selecting respective cells of the liquid crystal panel include segment signals which constitutes selection signals (scanning signals) and another segment signals indicative of display data. These driving signals are supplied as so-called alternating signals which have respective potentials thereof inverted periodically.
FIG. 14 is a schematic view for explaining a driving system of a passive matrix type liquid crystal panel which represents an STN (Super Twisted Nematic) type liquid crystal panel. The liquid crystal panel LCD forms pixels, that is, cells at portions where a plurality of common electrodes COM which are formed in the left-and-right direction in the drawing and a large number of segment electrodes SEG which are formed in the up-and-down direction intersect each other.
Scanning signals (common signals) are applied to respective common electrodes COM from a common scanning circuit and display signals (segment signals) are applied to respective segment electrodes SEG from a segment scanning circuit, thus enabling the pixels at the portions where both electrodes intersect each other to perform the display. A control circuit CONT generates display control signals in response to display signals, control signals and a power source supplied from external input terminals and applies given signals to a common scanning circuit COMD and a segment scanning circuit SEGD.
FIG. 15A to FIG. 15C are explanatory views of driving waveforms for the STN type liquid crystal panel of the related art, wherein FIG. 15A is a schematic view for explaining an example of an electrode arrangement structure of the common electrodes COM and the segment electrodes SEG which form the cells and FIG. 15B and FIG. 15C show waveform examples of the driving signals.
FIG. 16 is a circuit diagram for generating the driving waveforms shown in FIG. 15B and FIG. 17 is a circuit diagram for generating the driving waveforms shown in FIG. 15C.
In the electrode arrangement shown in FIG. 15A, the respective cells of the liquid crystal panel are formed by eighty pieces (first line to eightieth line) of common electrodes COM1, COM2, . . . COMn, COMn+1, . . . COM80 and 384 pieces of segment electrodes SEG1, SEG2, . . . SEGm, SEGm+1, . . . SEG384.
For example, focusing on the neighboring common electrodes COMn and COMn+1, with respect to the driving waveforms 1 of the conventional technique shown in FIG. 15B, the driving signals applied to the common electrodes COMn and COMn+1 are formed independently from each other.
That is, when the common electrode COMn of nth line is selected at a certain timing and thereafter the common electrode COMn+1 of (n+1)th line is selected at a next timing, a voltage supplied to the common electrode COMn is changed over from a selection voltage to a non-selection voltage, while a voltage supplied to the common electrode COMn+1 is changed over from a non-selection voltage to a selection voltage.
In an output circuit shown in FIG. 16 which outputs the driving waveforms 1 shown in FIG. 15B, Va indicates a first level voltage (high level) which is outputted from the control circuit CONT, Vb indicates a second level voltage (low level) which is outputted from the control circuit CONT, ca1, ca2, . . . caN indicate common electrode selection signals, and SWa1 and Swb1, Swa2 and Swb2, . . . SWaN and SwbN indicate a plurality of pairs of analogue switches which generate outputs ct1 to ctN to the common electrodes COM1, COM2, . . . COMn.
Although the common electrodes are indicated by COM1, COM2, . . . COMn, COMn+1, . . . COM80 in FIG. 15A, to simplify the explanation, the explanation is made hereinafter by indicating the common electrodes with 1 to n. Accordingly, the common electrode COMn indicates COMn, COMn+1, . . . COM80 shown in FIG. 14 in a representing manner.
A plurality of pairs of analogue switches SWa1 and Swb1, Swa2 and Swb2, . . . and SWaN and SWbN apply the output signals ct1 to ctN to the common electrodes 1 to n corresponding to the output signals ct1 to ctN by inputting the first level voltage Va, the second level voltage Vb and the common electrode selection signals ca1, ca2, . . . caN outputted from the control circuit CONT.
In performing the sequential changeover of such voltages, the voltage level of the selected common electrode COMn and that of the selected common electrode COMn+1 shown in FIG. 15B, for example, have polarities inverse to each other. At this point of changeover time, a state in which the charge of the common electrode COMn is fully discharged is established so that a given charge is applied to the common electrode COMn+1 from the non-selected level to the selected level without depending on the voltage level of the common electrode COMn.
Due to such a charging operation, the electric current is consumed so that the liquid crystal panel suffers from the undesired power consumption.
To improve this situation, as shown in FIG. 15C, there has been known a method in which the neighboring common electrodes COMn and COMn+1 are short-circuited at the time of changeover of the voltages.
FIG. 17 is a circuit diagram of a driving system shown in FIG. 15c for short-circuiting the neighboring common electrodes at the timing of changing over the common electrodes.
In FIG. 17, swc1 to swcN indicate analogue switches and cb1 to cbN indicate short-circuit signals for controlling the analogue switches swc1 to swcN, wherein symbols used in FIG. 17 which are as same as symbols used in FIG. 16 indicate parts having same functions.
In this system, as shown in FIG. 15C, when the currently selected common electrode is changed over to the non-selection state and the next common electrode is changed over to the selection state from the non-selection state, the currently selected common electrode and the next selected common electrode are short-circuited by the analogue switches swa1 to swcN.
With the provision of such a system, the neighboring common electrodes can hold the mean charge so that the charging current which is supplied to the next selected common electrode is reduced thus achieving the reduction of power consumption. The detailed operation of the above-mentioned output circuit is disclosed in Japanese Laid-open Patent Publication 194314/1999.
In the liquid crystal display device which uses this kind of liquid crystal panel, it is necessary to alternate the applied voltage so that the direct current voltage is not applied to the liquid crystal panel. FIG. 18 is a driving waveform chart for explaining problems derived from an alternation driving of a conventional technique. In the drawing, FLM indicates a frame signal, M indicates an alteration signal and CL1 indicates latching pulses. Symbols used in FIG. 18 which are as same as symbols used in FIG. 15 indicate parts having same functions.
As shown in FIG. 18, at the timing of the latching pulse CL1, a certain common electrode is selected, and the certain common electrode and another common electrode adjacent thereto (a common electrode to be selected next to the certain common electrode) are short-circuited at the timing of this selection.
However, for example, when the common electrode COMnxe2x88x921 and the COMn are short-circuited at the timing of the latch pulse n and the common electrode COMn and the COMn+1 are short-circuited at the timing of the latch pulse n+1, the consumptive electricity is increased on the contrary.
Due to the fact that the switching time cannot be made zero with respect to analogue switches, when the short-circuiting occurs, there arises a case in which both analogue switches are turned on. In the constitution of the output circuit described above as the related art, the common electrode is short-circuited with the common electrode to be selected next at the time that the polarities are changed due to the alternation, so long as the scanning line subjected to the common electrode is concerned, a voltage inverse to a voltage to be applied is applied and hence, the consumptive current reduction effect is suppressed. This has constituted one of problems to be solved by the present invention.
As described above, according to the above-mentioned related art, upon receiving the changeover signals for the common electrode selection signals, the terminals (common terminals) of the neighboring common electrodes are connected through the switching action and the charges of the neighboring common electrodes are held at the mean charge. Accordingly, with a given level voltage, the charging current supplied to the common electrode to be selected next can be substantially halved.
However, when the voltage applied to the liquid crystal is alternated, the processing at the time of alternation timing has not been sufficiently considered and hence, the reduction of the consumptive electricity has not been sufficient.
Further, there also exists the possibility of the occurrence of transitional consumptive current during the time required for switching operation of the output circuit.
The present invention has been made to solve following two problems (1), (2) which have not been solved by the related art.
(1) Due to the fact that the switching time of the analogue switches which are formed of a pair of CMS switching elements or the like constituting the common output part (common voltage output circuit) cannot be made zero, both switching elements are simultaneously turned on transitionally and hence, the consumptive electricity is increased.
(2) Since the voltage applied to the liquid crystal panel is of an AC voltage, the consumptive electricity is increased at the timing of alternation.
Accordingly, it is an object of the present invention to provide a liquid crystal display device which can further reduce the driving electricity for selection of cells by taking the operation time of analogue switches into consideration.
To achieve the above-mentioned object, in a liquid crystal display device including (a) a liquid crystal panel which arranges a plurality (for example, nN) of common electrodes extending in the first direction in parallel on one of a pair of substrates which sandwich a liquid crystal layer therebetween and arranges a plurality of segment electrodes extending in the second direction which intersects the first direction in parallel on the other of a pair of these substrates, (b) a common driver having a common voltage output part which applies scanning signals to a plurality of respective common electrodes, and (c) a segment driver having a segment voltage output part which applies data signals which correspond to display data to a plurality of respective segment electrodes, the common driver is configured to have following functions.
Function 1: In a period in which data signals are applied to a plurality of above-mentioned segment electrodes, selection periods are sequentially allocated to a plurality of respective common electrodes, and a selection voltage is applied to one of a plurality of common electrodes to which non-selection voltages are applied as scanning signals. The period in which the data signals are applied to a plurality of segment electrodes is also referred to as xe2x80x9cscanning periodxe2x80x9d. During this period, for example, the data signals are collectively supplied to respective segments which contribute to an image display among a plurality of segment electrodes. The allocation of selection periods is performed by inputting common electrode selection signals for selecting the given common electrode (of nMth line . . . 1xe2x89xa6nMxe2x89xa6nN) to a common driver from an external circuit, for example. In the specification of this application, although the common electrode selection signal is exemplified as CAM as will be explained later, M indicates a number (natural number of 2 or more) which indicates an order (time-sequential order) for selecting the given common electrode in a step for sequentially selecting a plurality of above-mentioned common electrodes (operation of the above-mentioned liquid crystal panel). M is not a value which always corresponds to a position (for example, the above-mentioned nMth line) of the given common electrode in the arrangement of a plurality of above-mentioned common electrodes (spatial or geometric arrangement in the liquid crystal panel). When the selection periods for selecting respective common electrodes are N times (N being a natural number) within the above-mentioned scanning period, the natural number M becomes also equal to or less than N.
Function 2: In applying the above-mentioned scanning signal to the given common electrode (hereinafter described as the nMth (spatial position) common electrode to which the Mth selection period (time-sequential order) is allocated among a plurality of above-mentioned common electrodes, the voltages are applied in the order that the application of the non-selection voltage is stopped at the first time at which the Mth selection period is started, the application of the selection voltage is started from the second time which comes after the first time, the application of the selection voltage is stopped at the third time at which the Mth selection period expires (coming after the second time), and the application of the non-selection voltage is started from the fourth time which comes after the third time. To explain the non-selection voltage (described as Vm later) in conjunction with a normally black type liquid crystal display device which makes light pass through a liquid crystal layer by applying an electric field to the liquid crystal layer as an example, the liquid crystal layer which is sandwiched between the common electrode to which the non-selection voltage is applied and the segment electrode which faces the common electrode in an opposed manner becomes a light shielding state so that pixels which correspond to the liquid crystal layer exhibits a so-called black display. To the contrary, when the selection voltage is applied to the common electrode, the liquid crystal layer which is sandwiched between the common electrode and the segment electrode which faces the common electrode in an opposed manner becomes a light transmitting state so that pixels which correspond to the liquid crystal layer exhibits a white display, for example. The selection voltage is comprised of a voltage (described as VL later) which has a potential lower than that of the above-mentioned non-selection voltage and a voltage (described as VH later) which has a potential higher than that of the above-mentioned non-selection voltage. By alternately applying either one of these voltages to a plurality of respective common electrodes at a given interval (INTERVAL), the polarization of the liquid crystal layer and the deterioration of the liquid crystal layer derived from the polarization can be prevented.
Function 3: The selection polarity of the selection voltage in the Mth selection period for the non-selection voltage and the polarity of the selection voltage which is applied to the nMth common electrode to which Mxe2x80x2th selection period (Mxe2x80x2 being a natural number which satisfies 1xe2x89xa6Mxe2x80x2 less than M) coming before the Mth selection period is allocated are compared each other. In the specification of the present application, when the selection voltage in the Mth selection period and the selection voltage in the Mxe2x80x2th selection period are both lower than the non-selection voltage (for example, when both selection voltages are VL) or when these voltages are both higher than the non-selection voltage (for example, when both selection voltages are VH), both selection voltages are defined as voltages having the same polarity, while either one of the selection voltage in the Mth selection period and the selection voltage in the Mxe2x80x2th selection period is lower than the non-selection voltage and the other selection voltage is higher than the non-selection voltage (for example, when one selection voltage being VL and the other selection voltage being VH), both selection voltages are defined as voltages having polarities opposite to each other. Here, Mxe2x80x2 which specifies the Mxe2x80x2th selection period which comes before the Mth selection period indicates a number (natural number) which indicates a time-sequential order for selecting the nMth common electrode in a step for sequentially selecting a plurality of above-mentioned common electrodes in the same manner as the above-mentioned M. Mxe2x80x2 is not a value which always corresponds to the given numbering nMxe2x80x2 (natural number) of the given common electrode in the spatial arrangement of a plurality of the above-mentioned common electrodes in the liquid crystal panel. Not only with respect to the liquid crystal display device but also with respect to a display device in general of a passive matrix driving system which uses organic EL elements or field emission type electron sources, the above-mentioned natural number Mxe2x80x2 can take arbitrary numerical values which are smaller than the natural number M. However, to focus on only the liquid crystal display device, it is desirable that the above-mentioned natural number Mxe2x80x2 satisfies the relationship [Mxe2x80x2=Mxe2x88x921] with respect to the above-mentioned natural number M. In the following explanation, the description is made by replacing the above-mentioned natural number Mxe2x80x2 with the natural number (Mxe2x88x921) and by adopting the present invention as a desirable application example to the liquid crystal display device. However, it becomes possible to replace the following natural number (Mxe2x88x921) with the natural number Mxe2x80x2. Further, the natural number nMxe2x80x2 is also expressed as the natural number nMxe2x88x921 hereinafter. Here, when a plurality of the above-mentioned common electrodes are constituted of nN pieces of common electrodes, the above-mentioned natural number nMxe2x80x2, can be set to an arbitrary value which falls in a range of [1xe2x89xa6nMxe2x80x2xe2x89xa6nN]. However, as described previously in conjunction with the function 1, the time-sequential order of the selection periods among the common electrodes and the geometric arrangement among display elements do not always agree with each other. Accordingly, it is impossible to univocally determine the magnitude relationship between nMxe2x80x2 and nM and the magnitude relationship between nMxe2x88x921 and nM.
Function 4: In comparison with the function 3, when it is judged that the selection voltage in the Mth selection period and the selection voltage in the (Mxe2x88x921)th selection period have the same polarity, a portion of the common voltage output part which corresponds to the nMth common electrode and a portion of the common voltage output part which corresponds to the nMxe2x88x921th common electrode are short-circuited during the period between the first time and the second time.
Function 5: In comparison with the function 3, when it is judged that the selection voltage in the Mth selection period and the selection voltage in the (Mxe2x88x921)th selection period have the reversed polarities, during the period between the first time and the second time, a portion of the common voltage output part which corresponds to the nMth common electrode and a portion of the common voltage output part which corresponds to the nMxe2x88x921th common electrode are not short-circuited.
In the liquid crystal display device of the present invention which is characterized by the above-mentioned functions, the supply of the selection voltage to the nMth common electrode to which the Mth selection period is allocated is started at the second time which comes after the first time at which the common electrode selection signal which selects the nMth common electrode is generated. Further, the supply of the selection voltage to the nMth common electrode is finished at the fourth time which comes after the third time at which the common electrode selection signal which selects the nMth common electrode is extinguished. However, due to the function 4 of the present invention, when the selection voltage which is supplied to the nMth common electrode and the selection voltage which is applied to the nMxe2x88x921th common electrode in the preceding selection period have the same polarity, the charge remaining in the nMxe2x88x921th common electrode which has finished the selection period is processed by short-circuiting the common voltage output part (one of ct1 to ctN which will be explained later, here referred to as ct(Mxe2x88x921)) which is connected to the nMxe2x88x921th common electrode and the common voltage output part (the other one of ct1 to ctN which will be explained later, here referred to as ctM) which is connected to the nMth common electrode. Here, the respective potentials of the common voltage output part ct(Mxe2x88x921) and the common voltage output part ctM are changed to the mean value between the selection voltage and the non-selection voltage. Accordingly, the charge corresponding to the potential of the mean value is preliminarily supplied to the nMth common electrode from the nMxe2x88x921th common electrode. Accordingly, the undesired charge in the nMxe2x88x921th common electrode is effectively used for charging the nMth common electrode and hence, the electricity which is consumed at the time of selecting the cell (pixel) corresponding to the nMth common electrode can be saved by such a charged amount. Here, the short-circuiting of the common voltage output part ct(Mxe2x88x921) and the common voltage output part ctM is performed by providing a short-circuiting line which short-circuits both common voltage output parts and by mounting a switching element (swcM which will be explained later) on the short-circuiting line.
On the other hand, due to the above-mentioned function 5 of the present invention, when the polarity of the selection voltage which is supplied to the nMth common electrode and the polarity of the selection voltage applied to the nMxe2x88x921th common electrode in the preceding selection period are opposite to each other, the common voltage output part ct(Mxe2x88x921) and the common voltage output part ctM are not short-circuited. This is because that when the potential of the nMth common electrode is changed to the mean value between the selection voltage and the non-selection voltage of the nMxe2x88x921th common electrode due to the short-circuiting of both common electrodes, the supply of charge for compensating for the change amount of the mean value becomes necessary newly at the time of setting the potential of the nMth common electrode to the selection voltage during the selection period. This reason is also substantiated by the fact that when the selection voltage to be supplied to the nMth common electrode charges electrons as the charge to the common electrode, the selection voltage of the nMxe2x88x921th common electrode which has the polarity opposite to the polarity of the nMth common electrode charges holes as the charge to the common electrode (to reduce the density of electrons of the common electrode). Accordingly, the function 5 of the present invention also can contribute to the reduction of the electricity consumed by the selection of the cell corresponding to the nMth common electrode.
In the present invention, for the selection start time of the nMth common electrode (the above-mentioned first time), the time for applying the selection voltage through a power source line (the above-mentioned second time) to this is delayed by a given time and the charge is introduced into the nMth common electrode from the separate common electrode in such a given time. Further, in the present invention, for the selection finish time of the nMth common electrode (the above-mentioned third time), the time for applying the non-selection voltage through a power source line (the above-mentioned fourth time) to this is delayed by a given time and the charge remaining in the nM th common electrode is distributed to a separate common electrode in such a given time. Accordingly, apart from the selection signal CAM of the nMth common electrode, it is required that a control signal which is delayed at a given interval is generated and, in response to the control signal, the nMth common electrode and the power source line which supplies the selection voltage are connected with each other at the second time, and the nMth common electrode and a power source line which supplies the non-selection voltage are connected at the fourth time. In this case, it is preferable to generate this control signal (referred to as CAMxe2x88x92later) by delaying the selection signal for the nMth common electrode. Further, the control signal which makes a switching element provided to a short-circuiting line which short-circuits the above-mentioned common voltage output parts conductive (referred to as CAMxe2x80x2 later, wherein Mxe2x80x2 included in CAMxe2x80x2 differs from the above-mentioned natural number Mxe2x80x2) and the control signal (referred to as CAMxe2x80x3 later) which interrupts the switching element may be generated by delaying the selection signal of the nMth common electrode. These three kinds of control signals can be accurately generated using a simple circuit constitution by arranging delay circuit blocks in three stages at the rear stage of the output part of the selection signal CAM to the nMth common electrode, wherein the control signal CAMxe2x80x2 which makes the switching element of the short-circuiting line conductive is taken from the first-stage output, the control signal CAMxe2x80x3 which interrupts the switching element of the short-circuiting line is taken from the second-stage output, and the selection signal CAM of the nMth common electrode and the AND condition are taken from the output of the third stage so that the nMth common electrode and the power source line of the selection voltage are communicated with each other, and the selection signal CAM and the NOR (NOTOR) condition are taken so as to generate respective control signals CAMxe2x88x92which make the nMth common electrode and the power source line of the non-selection voltage communicate with each other.
In any liquid crystal display devices of the present invention which have been explained heretofore, it becomes possible to drastically reduce the consumptive electricity which is necessary at the time of performing a so-called alternation timing driving which operates the liquid crystal display device by inverting the polarity of the selection voltage applied to the common electrode with respect to the non-selection voltage compared with the conventional liquid crystal display device. As a result, by mounting the liquid crystal display device of the present invention on the portable telephone, the portable information terminal or the like which has the small battery capacitance, the operable time which corresponds to the charging time of such equipment can be prolonged. To take the portable telephone as an example, a product having the same charge capacitance and the same standby time (product assurance value) with a conventional product can be constituted far lighter than the conventional product.
In the above-mentioned liquid crystal display device according to the present invention, it is preferable that the common voltage output part which corresponds to the nMth common electrode and the common voltage output part which corresponds to the nMxe2x88x921th common electrode are configured to be short-circuited at the time after the first time and before the second time. Further, in the liquid crystal display device according to the present invention, it is preferable that the period in which the common voltage output part which corresponds to the nMth common electrode and the common voltage output part which corresponds to the nMxe2x88x921th common electrode are short-circuited expire are configured to after the third time and before the fourth time. In this manner, by delaying the timing of the starting of short-circuiting between the common voltage output parts more than the first time or the third time and by advancing the timing of the finishing of short-circuiting between the common voltage output parts more than the second time or the fourth time, it becomes possible to surely prevent the mixing of noise voltage into the power source line due to the unexpected delay of the open/close timing of the switching element provided between the common voltage output parts and the power source line which supplies the selection voltage or the non-selection voltage.
Further, in the above-mentioned liquid crystal panel which constitutes the liquid crystal display device of the present invention, the nMth common electrode and the nMxe2x88x921th common electrode may be arranged adjacent to each other (spatially) or the nMth common electrode and the nMxe2x88x921th common electrode may be arranged in a spaced-apart manner. This is substantiated by the explanation of the function 1 and the function 3 of the liquid crystal display device of the present invention. That is, as has been explained by taking the nMth common electrode and the nMxe2x88x921th common electrode as examples, the order of the respective selection period of a plurality of common electrodes is not constrained by the order of the spatial arrangement thereof. Here, the constitution which arranges the nMth common electrode and the nMxe2x88x921th common electrode in the spaced-apart manner is put into practice in the liquid crystal display device or the like which adopts a toggle type driver as a common driver.
The liquid crystal display devices whose summaries have been explained heretofore are not limited to the above-mentioned constitutions and constitutions of embodiments which will be explained hereiafter and it is needless to say that various modifications can be made without departing from the technical concept of the present invention.